[Filebeat] Skip flaky windows tests

`test_clean_inactive` (elastic#8102) and `test_clean_removed` (elastic#7690) have been flaky recently again on Windows. Skipping these test for Windows.

filebeat/tests/system/test_registrar.py

@@ -773,6 +773,7 @@ def test_state_after_rotation_ignore_older(self):
             assert self.get_registry_entry_by_path(os.path.abspath(testfile_path1))["offset"] == 9
             assert self.get_registry_entry_by_path(os.path.abspath(testfile_path2))["offset"] == 8
 
+    @unittest.skipIf(os.name == 'nt', 'flaky test https://github.com/elastic/beats/issues/8102')
     def test_clean_inactive(self):
         """
         Checks that states are properly removed after clean_inactive
@@ -819,6 +820,7 @@ def test_clean_inactive(self):
         # Make sure the last file in the registry is the correct one and has the correct offset
         assert data[0]["offset"] == self.input_logs.size(file3)
 
+    @unittest.skipIf(os.name == 'nt', 'flaky test https://github.com/elastic/beats/issues/7690')
     def test_clean_removed(self):
         """
         Checks that files which were removed, the state is removed